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   /*
    * Copyright 2016 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.resolver;
  
  import io.netty.util.concurrent.EventExecutor;
  import io.netty.util.concurrent.Future;
  import io.netty.util.concurrent.FutureListener;
  import io.netty.util.concurrent.Promise;
  import io.netty.util.internal.PlatformDependent;
  
  import java.net.InetAddress;
  import java.net.InetSocketAddress;
  import java.net.UnknownHostException;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;
  
  /**
   * A {@link NameResolver} that resolves {@link InetAddress} and force Round Robin by choosing a single address
   * randomly in {@link #resolve(String)} and {@link #resolve(String, Promise)}
   * if multiple are returned by the {@link NameResolver}.
   * Use {@link #asAddressResolver()} to create a {@link InetSocketAddress} resolver
   */
  public class RoundRobinInetAddressResolver extends InetNameResolver {
      private final NameResolver<InetAddress> nameResolver;
  
      /**
       * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned by
       * {@link #resolve(String)}
       * @param nameResolver the {@link NameResolver} used for name resolution
       */
      public RoundRobinInetAddressResolver(EventExecutor executor, NameResolver<InetAddress> nameResolver) {
          super(executor);
          this.nameResolver = nameResolver;
      }
  
      @Override
      protected void doResolve(final String inetHost, final Promise<InetAddress> promise) throws Exception {
          // hijack the doResolve request, but do a doResolveAll request under the hood.
          // Note that InetSocketAddress.getHostName() will never incur a reverse lookup here,
          // because an unresolved address always has a host name.
          nameResolver.resolveAll(inetHost).addListener(new FutureListener<List<InetAddress>>() {
              @Override
              public void operationComplete(Future<List<InetAddress>> future) throws Exception {
                  if (future.isSuccess()) {
                      List<InetAddress> inetAddresses = future.getNow();
                      int numAddresses = inetAddresses.size();
                      if (numAddresses > 0) {
                          // if there are multiple addresses: we shall pick one by one
                          // to support the round robin distribution
                          promise.setSuccess(inetAddresses.get(randomIndex(numAddresses)));
                      } else {
                          promise.setFailure(new UnknownHostException(inetHost));
                      }
                  } else {
                      promise.setFailure(future.cause());
                  }
              }
          });
      }
  
      @Override
      protected void doResolveAll(String inetHost, final Promise<List<InetAddress>> promise) throws Exception {
          nameResolver.resolveAll(inetHost).addListener(new FutureListener<List<InetAddress>>() {
              @Override
              public void operationComplete(Future<List<InetAddress>> future) throws Exception {
                  if (future.isSuccess()) {
                      List<InetAddress> inetAddresses = future.getNow();
                      if (!inetAddresses.isEmpty()) {
                          // create a copy to make sure that it's modifiable random access collection
                          List<InetAddress> result = new ArrayList<InetAddress>(inetAddresses);
                          // rotate by different distance each time to force round robin distribution
                          Collections.rotate(result, randomIndex(inetAddresses.size()));
                          promise.setSuccess(result);
                      } else {
                          promise.setSuccess(inetAddresses);
                      }
                  } else {
                      promise.setFailure(future.cause());
                  }
              }
          });
      }
  
      private static int randomIndex(int numAddresses) {
          return numAddresses == 1 ? 0 : PlatformDependent.threadLocalRandom().nextInt(numAddresses);
     }
 
     @Override
     public void close() {
         nameResolver.close();
     }
 }